Herpes Simplex Virus Type 1 (HSV-1) is the cause of primary and recurrent infections of the mouth, gums, and lip, genital and ocular infections, generalized infections in neonates and immunodeficient individuals, and rare cases of encephalitis. The closely related virus HSV-2 is the cause of genital herpetic lesions and numerous neonatal infections. The process by which HSV-1 absorbs to and enters cells is not well understood. An understanding of this process may well lead to the development of effective and practical antiviral agents and/or immunization methods that block these early stages of infection. Several HSV-1 glycoproteins (gB, gC, gD, and gH) have been proven or are likely to be involved in these processes. Initial interaction of HSV-1 virions with the cell surface may be mediated by binding of viral glycoproteins to cell surface heparan sulfate. The identity of the viral adsorption protein that interacts with heparan sulfate is unclear, since virions lacking gB, gC, or gD all adsorb to cells. However, solubilized gB and gC bind to heparin. It has been suggested that initial binding to cells may be mediated by either glycoprotein. This hypothesis will be tested directly by the production of virions lacking both gB and gC and their use in adsorption experiments. Virions deficit in other combinations of glycoproteins will be tested as necessary. These virions will be produced by introduction of sites recognized by Factor Xa, a highly specific protease, or by recombination of existing glycoprotein deficient mutants. Once adsorbed, virions penetrate by fusion of the virion envelope with the plasma membrane. Three glycoproteins, gB, gC, and gH, have been shown to be essential for this process, although the exact roles of each are unknown. The penetration process is likely to be related to the induction of cell fusion by syncytial mutants of HSV-1, since many of the same glycoproteins have been implicated in each. Also involved in cell fusion is the UL53/syn1 gene. We have performed an extensive in vitro analysis on the protein specific by this gene. Significantly, we have found that the processed from of this protein has a hydrophobic N-terminal end, similar to the hydrophobic N- terminal fusion domains of other viral fusion proteins. It is likely that UL53 acts as a membrane fusogen in the cell fusion process and possibly in penetration as well. Extensive additional characterization of UL53 is proposed to augment our understanding of this molecule. These studies will include determination of the transmembrane structure of the UL53 protein, use of UL53 specific antisera to characterize the protein in vivo, sequencing of additional UL53 syncytial mutants, and production of new UL53 mutants. Finally, a transfection system for induction of syncytium formation by HSV-1 will be developed and used to identify the viral genes that are necessary and sufficient for cell fusion. This will eventually make it possible to probe the mechanism of cell fusion and determine its relationship to virion penetration.